In a conventional cellular communication system, the arrangement of network elements is hierarchical and standardised. For example, for a conventional UTRAN (UMTS (Universal Mobile Telecommunications System) Terrestrial Radio Access Network), a hierarchical arrangement of Mobile Switching Centres (MSCs), Radio Network Controllers (RNCs) and base stations (Node Bs) is implemented to support the mobile segment.
In such systems, there is an inherent and/or practical limitation on the number of elements at one level of the hierarchy and the number of elements at the next level. For example one MSC-U (Mobile Switching Centre for UMTS) can typically connect up to ten or twenty RNCs and one RNC can typically manage tens (or hundreds) of Node Bs (UMTS base stations).
In such a conventional arrangement, mobile user equipments make handovers from cell to cell as they move within a UMTS network and occasionally such a handover is across the boundary between two sets of cells controlled by different RNCs. In this case, user equipments that currently have a dedicated connection to the network (i.e. they are in a voice or data call) will be involved in a relocation procedure that moves the management of the call from the current (source) RNC to a new (target) RNC. This procedure, known as SRNS (Serving Radio Network Subsystem) relocation, is complex and can lead to an interruption in service for either a data or a voice call. In the case of a voice call this can be particularly noticeable to the participants in the call and can cause a disruption or delay in the perceived speech. Also, a significantly greater amount of RNC and MSC-U signalling messaging is required to achieve an SRNS relocation than for a handover between cells controlled by the same parent RNC.
A method which has been used to increase the capacity of cellular communication systems is the concept of hierarchical cells wherein a macrocell layer is underlayed by a layer of typically smaller cells having coverage areas within the coverage area of the macrocell. Such microcells, picocells and femtocells have much smaller coverage thereby allowing a much closer reuse of resources.
Currently there is a trend towards introducing a large number of small picocells to UMTS systems. For example, it is envisaged that picocell base stations known as residential access points may be deployed typically having only a target coverage area of a single residential dwelling or house. As another example, it has been proposed to cover e.g. office buildings in a number of small picocells with a range of a few tens of meters.
Furthermore, it has been proposed that individual residential or enterprise access points include at least some RNC functionality such that the individual access point is coupled to the network as an RNC entity with an individual RNC identity. Such an approach may provide the advantage that RNC devices supporting a very large number of subordinate base station/Node B access points will not be required. Also the standard interface between a Node B and RNC is delay sensitive and would not lend itself to some of the potential deployment environments for a home Node B access point, for example an ADSL (Asymmetric Digital Subscriber Line) connection between the access point and RNC.
Thus, in such networks, a large number of RNC entities will be present in the network (e.g. in some architectures there may be one RNC for each access point). However, such an approach is also associated with some disadvantages.
For example, mobile user equipments supported by such a network of access points will experience an SRNS relocation at every handover between access points. Thus, the SRNS relocation is no longer a rare situation as in a conventional UMTS network but occurs very frequently. As a consequence, a large additional signalling overhead is introduced and the quality of the provided communication service is reduced. Thus, whereas a collapsed architecture with RNC functionality in each access point may result in reduced infrastructure cost (by removing the need for a dedicated RNC), it results in potential issues of degraded quality (in particular for voice handovers) and increased resource required for a significantly increased signalling overhead (due to the higher number of relocations performed).
Furthermore, the processing resource required in each access point can be significantly increased resulting in increased cost and complexity of the access point. The processing resource available at the access point must be dimensioned such that it is sufficient to provide both the base station functionality and RNC functionality for the worst case which typically results in a large resource overhead.
Hence, an improved system would be advantageous and in particular a system allowing increased flexibility, improved resource usage, reduced signalling, improved communication quality, reduced cost and/or complexity of elements and/or improved performance would be advantageous.